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USMLE - Pharm

Kaplan Section 1 Chapter 1 Pharmacokinetics

What is the pKa? pH at which the drug or any compound is 50% ionized (doesn't cross membranes, but better cleared from the body) and 50% nonionized (crosses membranes)
Weak acids are better cleared in which environment? Slightly acidic or slightly basic? Slightly basic because in a more basic environment, weak acids will be ionized and therefore, not cross membranes as easily.
Weak bases are better cleared in which environment? Slightly acidic or slightly basic? Slightly acidic because in a slightly acidic environment, weak bases are protonated (ionized) and therefore doesn't cross membranes as easily.
Henderson Hasselbalch equation for weak acids pH - pKa = log ([ionized]/[nonionized])
Henderson Hasselbalch equation for weak bases pH - pKa = log ([nonionized]/[ionized])
T or F: both bound and unbound drugs are filtered F. Only unbound drug is filtered in the kidney.
T or F: Both ionized and nonionized forms of a drug are filtered. True
Acidification of urine increases renal clearance of what type of drugs? Weak bases
Alkalinization of urine increases renal clearance of what type of drugs? Weak acids
What percentage of a drug is absorbed if it is administered via IV? no absorption involved because IV --> therefore, no loss of blood.
What is the bioavailability of an IV bolus of a drug? 100% bioavailability, f = 1
T or F: Only unbound drugs cross biomembranes T.
What types of drugs can cross the BBB? lipid soluble or very low molecular weight
What is volume of distribution (Vd)? How much of a drug actually gets through the plasma, into the tissues.
What does a high Vd mean? More drug made it into the tissues, and therefore, less left in the plasma.
what drugs cause SLE-like syndromes? In what type of people do they occur? Hydralazine > Procainamide > INH. In slow acetylators (can't break down/conjugate the drug).  
What drug is the most common cause of fulminant hepatitis? When would this happen? Acetominophen. When GSH (glutathione) is depleted in the liver -- you need glutathione to conjugate/break down acetominophen.  
What does t1/2 (half life) really mean? Time it takes to eliminate 50% of a given amount of drug
What does it mean when a drug has a zero order elimination rate? Give an example Rate of elimination is indep of the [ ] of the drug in the plasma (or amnt of drug already in the body). CONSTANT amnt of drug is eliminated per unit time. Examples include: EtOH, phenytoin (high therapeutic doses), and salicylates (high toxic doses)
What is the t1/2 of a drug that has zero order elimination? No fixed t1/2 because a constant amount is eliminated per unit time. t1/2 depends on how much drug you gave in the first place
What is the t1/2 of a drug that has first order elimination? t1/2 is CONSTANT
What does it mean when a drug has first order elimination? the rate of elimination is proportional to the [ ] of drug already in the plasma. (the higher the [ ], the faster the elimination)
What can you say about the clearance of a drug that has first order elimination? Clearance is CONSTANT
Formula to find a dose of a first order drug Dose = Vd x C0. Vd is volume of distribution, C0 is plasma [ ] at time 0. CO VetoeD the DOSE.
Formula to find the t1/2 of a first order drug t1/2 = 0.7/k (k is first order rate constant of elimination)
What forms of a drug are filtered? Ionized, nonionized, and unbound drug (bound forms of drugs do NOT get filtered)
What is renal clearance? Volume of blood cleared of the drug per unit time. (volume of blood that is cleared of the blood)/(time)
What are the conditions under which renal clearance = GFR (glomerular filtration rate)? If there is no active secretion or reabsorption. (ie. Inulin)
What is a normal GFR? 120 mL/min
What is the formula for clearance? Cl = k (elim constant) x Vd (volume of distribution). Cleared means OKed and not VetoeD.
What is the formula for rate of infusion of a drug? Infusion rate = Cl (clearance) x Css ([ ] of drug in the steady state). Clara and Cassie were INFUriated with the infusion of coloreds into their school.
Formula for loading dose LD = Vd (vol of distrib) x Css ([ ] drug in steady state). Laura DG VetoeD Cassie from the club.
Formula for maintenance dose MD = clearance x Css ([ ] drug in steady state) x dosing interval. Clara, Cassie, and DosiDo graduated with their MDs.
What does steady state for a drug mean? that rate in = rate out.
What is the plateau principle for a drug? Time to reach steady state is dependent only on the t1/2 of a drug and is independent of dose size and frequency of administration
How many half lives does it take a first order drug to reach clinical steady state? 4 to 5 (95% of [ ] of the drug in steady state)
For a first order drug, how does the amount of time needed to reach steady state change as you increase the rate of infusion? Takes the same amount of time to reach steady state. The difference is that with a higher rate of infusion, you get a greater concentration of the drug in the plasma at steady state. Steady state is simply what goes in = what is eliminated.
If doses of a drug are to be administered at each half life of the drug, then what should the loading dose be? Loading dose should then be double the amount of the dose used for maintenance.
Formula for renal clearance of a protein-bound drug Cl(renal) = GFR x free fraction
What is the Vd if the drug is distributed into total plasma volume? 3 L
What is the Vd if the drug is distributed into total blood volume? 5 L
What is the Vd if the drug is distributed into extracellular fluid? 12-14 L
What is the Vd if the drug is distributed into total body water? 40-42 L
Which of these inhibits CYP450? Carbamazepine, EtOH, Ketoconazole, Phenobarbital, Rifampin Ketoconazole is the only inhibitor. All the rest are inducers.
A 500mg dose of a drug has therapeutic efficacy for 6 hr. If the half-life of the drug is 8 hr, for how long would a 1g dose be effective? Drug stays in therapeutic range for 6 hours so the therapeutic level is somewhere above 250 mg. With 1 g dose, after 8 hours, the [ ] will still be 500mg, so the drug will be efficacious for 8 hr, then 6 additional hr, so the total is 14 hr.
Created by: christinapham



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